The original joke is that Kyon says ka ni kamareteru. ka is "mosquito", and ni means "by" in this case.
Mikuru says kani-shan (san) ni kamareta since she misunderstood ka ni as one word, kani, or "crab". So she literally says she was bitten by a crab. Koizumi apparently knows what she means though.
Damn, talk about itchyThese mosquitoes are eating me aliveCopycat*scratch-scratch**scratch-scratch*Koijumi-kunI got bit by CheetosA mosquito bit you, huh?Let's put some anti-itching cream on you.